EP0543014B1

EP0543014B1 - Six-stage rolling mill

Info

Publication number: EP0543014B1
Application number: EP92910178A
Authority: EP
Inventors: Toshiki Kawasaki Steel Corporation Hiruta; Kunio Kawasaki Steel Corporation Kitamura; Ikuo Kawasaki Steel Corporation Yarita
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1991-05-16
Filing date: 1992-05-18
Publication date: 1998-08-19
Anticipated expiration: 2012-05-18
Also published as: JPH05245506A; KR930701244A; KR100216299B1; DE69226690D1; EP0543014A1; EP0543014A4; DE69226690T2; DE69226690T3; EP0543014B2; CA2087156A1; WO1992020471A1; JP2654313B2; CA2087156C

Description

Technical Field

This invention relates to a hot rolling mill and is concerned with a hot finish rolling mill for hot rolling a sheet bar rolled by a rough rolling mill to a product thickness or to a six high rolling mill for cold rolling strip sheet rolled by a hot finish rolling mill, in particular, to precisely control the sheet crown which is defined as the difference in the sheet thickness between a central portion of the sheet width and portions in the vicinity of edges, thereby preventing the sheet edges from becoming extremely thin because of edge drop.

Background art

Generally, when a hot rolled steel sheet is produced by means of a hot finish rolling mill, the rolls of the mill are deflected due to the rolling load, thereby the sheet thickness at the central portion of the sheet width becomes greater than the sheet thickness at portions in the vicinity of the opposite edges of the rolled sheet, that is a sheet crown is formed in the rolled sheet. By the way, the sheet crown, if the sheet crown becomes large, makes it difficult to provide an adequate sheet profile when cold rolling in the next step, which also results in shape deficiency and unavoidably results in a reduction in yield. Thus it is required for the hot finish rolling mill to make the sheet crown as small as possible.

Thus, for the purpose of controlling the shape of sheet to reduce the sheet crown, for example, JP-B-62-10722 discloses a six high rolling mill to be installed in a post-stage stand, wherein a rolling mill array includes intermediate rolls having a constant diameter over the full length thereof arranged between backup rolls and work rolls, respectively, and these intermediate rolls are adapted to shift in mutually opposite axial directions, whereby the ability to control the sheet crown is enhanced. Furthermore, JP-A-57-91807 discloses a rolling mill in which an S-shaped crown is formed on any one of a work roll, an intermediate roll or a backup roll, and the roll having the S-shaped crown is shifted in the axial direction, whereby the ability for controlling the sheet crown is enhanced.

However, in the former prior art disclosed in JP-B-62-10722, the length of the intermediate roll is made approximately the same as the lengths of the backup roll and the work roll, so that when the intermediate roll is shifted in order to make the sheet crown small, the length of contact of the intermediate roll with the backup roll and the work roll becomes short, and the mill rigidity of the rolling mill decreases. Hence there has been the problem that, when the rolling load changes due to temperature deviation in the sheet bar or the like, the roll gap between the pair of work rolls greatly changes, and no predetermined accuracy in the sheet thickness can be provided. There has been such a problem that, when the center of the sheet in the width direction deviates from the center of the rolling mill due to deviation of the sheet bar or the like, meanderings resulting from the difference in rigidity of the right and left portions of the rolling mill occur and sometimes it becomes impossible to carry out rolling because of reduction ears caused by miss rolling.

In addition, there has been the problem that spalling occurs on the surfaces of the rolls resulting from the increase in pressure between the rolls on account of the short length of contact of the intermediate roll, and the service life of the rolls decreases.

It is noted that the problem mentioned above can be avoided by decreasing the shift amount of the intermediate rolls, but the ability to control the crown of the work rolls in the rolling mill is then greatly limited.

Also in the later prior art disclosed in JP-A-57-91807, there has been the problem that, when profile control is performed by shifting intermediate rolls provided with an S-shaped crown, the control of crown becomes impossible due to the abrasion of the rolls.

Furthermore, when profile control is performed by providing a curved roll crown on the intermediate roll or the backup roll, it becomes necessary to enlarge the roll crown in order to ensure a large control amount for the crown, but when a sheet bar having a relatively narrow width is rolled with small rolling load by providing such a large roll crown, non-contact portions are generated between the backup roll and the intermediate roll or between the backup roll and the work roll, and the mill rigidity of the rolling mill becomes low, which unavoidably results in a decrease in the accuracy of the sheet thickness. In addition, there has been the problem that, when the non-contact portions are generated, meander and reduction ears occur in the rolled sheet as a result of the difference in rigidity in the axial direction of the rolls and as a result rolling of the sheet sometimes becomes impossible.

Disclosure of the Invention

This invention solves all such problems in the prior art and provides a six high rolling mill adapted for controlling both the sheet crown and edge drop of the sheets to prevent a decrease in mill rigidity of the rolling mill and meander of the sheet resulting from great shifting of the intermediate roll and to attain an increase in service life of the rolls.

According to the present invention there is provided a six high rolling mill comprising upper and lower work rolls, a pair of intermediate rolls and a pair of backup rolls wherein at least the intermediate rolls and the work rolls are adapted to be shifted in their axial directions, said intermediate rolls are provided with roll crowns which are in a point symmetrical relationship with reference to the centre point of the mill, and said work rolls have roll profiles which are in a point symmetrical relationship with reference to the centre point of the mill characterised in that the roll profile of one of the intermediate rolls is expressed by the following ternary equation (1): y1(x)= -a[{x-(δ+OF)}/L]3+b(x/L)

where y₁ is the generating line of the crown of the roll,

a is a coefficient of the third order,

b is a coefficient of the first order,

x is the coordinate of the barrel center,

L is 1/2 of the barrel length of the intermediate roll,

δ is the shift amount of the intermediate roll relative to a start point where x=LB, and

OF is the offset amount in the axial direction; in that the roll profile of the other of the intermediate rolls is expressed by the following ternary equation (2): y2(x)= -a[{x+(δ+OF)}/L]3+b(x/L)

where y₂ is the generating line of the crown of the roll; and in that each of the intermediate rolls has a barrel length 1.2-2.5 times longer than that of its backup roll such that the intermediate rolls always contact the backup rolls over the full length thereof at the maximum and minimum shifted positions of the intermediate rolls.

In a preferred embodiment of the present invention, the barrel length of the work roll is longer than that of the intermediate roll and preferably 1.4 to 2.5 times longer than that of the intermediate roll.

The work roll may be provided with a roll crown having a shape such as a one side taper shape where the barrel diameter is gradually reduced towards one end of the roll barrel or a two side taper shape where the barrel diameter is gradually reduced towards the opposite ends from the center of the barrel length.

The six high rolling mill according to the invention is able to reduce the load affected between rolls, in particular, the barrel end portions of the intermediate and work rolls by providing the roll crown for the intermediate rolls, thereby improving the ability to control the crown. Particularly, the "S" shaped roll crown can effectively reduce the rolling load applied on both edge portions of the sheet, and when the intermediate rolls are respectively shifted in the opposite directions relative to each other in the point symmetry relationship, the aforementioned function is more remarkably attained and as a result a greater crown control ability can be attained.

In the rolling mill according to the invention, since the intermediate roll has a barrel length longer than that of the backup roll as mentioned above, even if the intermediate roll is greatly shifted, the intermediate roll can always effectively contact the backup roll over the full length thereof so that the mill rigidity of the rolling mill is effectively prevented from decreasing due to profile control. Therefore accuracy of the sheet thickness is greatly improved without being affected by variation in the width of the sheet to be rolled. Furthermore, even if the sheet to be rolled has camber, the sheet is subjected to uniform reduction through the whole sheet width so that the occurence of meander can be effectively reduced.

It should be noted that if the roll barrel of the intermediate roll had a length which was the same as the roll barrel of the backup roll, it would be necessary to use a large roll crown so as to provide a large difference between the maximum diameter and minimum diameter of the roll barrel of the intermediate roll in order to attain the necessary crown control. As a result, the contact pressure generated between contacting rolls increases and causes spalling on the surfaces of the rolls and also reduce the service life of the rolls. Furthermore, when the sheet bar has a relatively narrow width and the rolling load is small, non-contact portions are generated between the barrels of the intermediate and backup rolls or between the barrels of the intermediate and work rolls. Thus, the mill rigidity of the rolling mill reduces and as a result , the desired accuracy of the sheet thickness can not be obtained. Therefore, in order to remove the aforementioned problems, the barrel length of the intermediate roll is 1.2∼2.5 times as long as the barrel length of the back up roll.

Furthermore, the barrel length of each work roll is preferably longer than that of the intermediate roll, and advantageously the line barrel length of the work roll is 1.4∼2.5 times as long as the intermediate roll so that the work roll always effectively contacts the intermediate roll in spite of shifting of the intermediate roll to improve the mill rigidity of the rolling mill and particularly reduce meandering of the sheet. Moreover, the service life of the roll is improved by increasing the contact range between the rolls and by preventing the contact pressure between the rolls from increasing.

Brief Description of Drawing

Fig. 1 is a schematic front view of a rolling mill according to the present invention;

Fig. 2 is a diagrammatic view illustrating the roll crown for an intermediate roll of the mill of Fig. 1;

Fig. 3 is a schematic view illustrating the intermediate rolls of Fig. 1 in their in shifted positions;

Fig. 4 is a block diagram of a control system of the rolling mill of Fig. 1;

Fig. 5 shows graphs showing the relationship between the pressure between the rolls of the mill and the sheet crown;

Fig. 6 is a graph showing the relationship between the ratio of the barrel length of the intermediate and backup rolls of the mill and the maximum pressure between rolls;

Fig. 7 is a graph showing the contact conditions between the rolls of the mill with respect to the ratio of the barrel length of the intermediate and backup rolls;

Fig. 8 is a diagrammatic view illustrating bending of the intermediate rolls of the mill;

Fig. 9 is a graph showing the relationship between the ratio of the barrel length of the intermediate and backup rolls of the mill and the deflection amount of the intermediate rolls;

Fig. 10 is a graph showing the distribution of sheet crown with respect to the number of rolled sheets;

Fig. 11 is a diagrammatic side view of a mill of the invention illustrating the supply of lubricant;

Fig. 12 is a diagrammatic front view of the mill of Fig. 11;

Fig. 13 is a graph showing the relationship between the diameter of the work rolls and the crown control amount;

Fig. 14 is a schematic front view illustrating a rolling mill of the invention;

Fig. 15 is a graph showing the distribution of sheet crown with respect to the number of rolled sheets;

Fig. 16 is a graph showing the amount of edge drops occuring;

Fig. 17 is a schematic front view illustrating a rolling mill of the invention;

Fig. 18 is a diagrammatic view illustrating a mill of the invention with the work rolls in the shifted position;

Fig. 19 is a graph showing the variation of edge drop;

Fig. 20 is a graph showing the distribution of sheet crown with respect to the number of rolled sheets;

Fig. 21 is a graph showing the amount of edge drop occuring;

Fig. 22 is a schematic front view illustrating a rolling mill of the invention;

Fig. 23 is a graph showing the distribution of sheet crown with respect to the number of rolled sheets;

Fig. 24 is a schematic front view illustrating a rolling mill of the invention;

Fig. 25 is a graph showing the distribution of sheet crown with respect to the number of rolled sheets;

Fig. 26 is a schematic front view illustrating a rolling mill of the invention;

Fig. 27 is a graph showing the distribution of sheet crown with respect to the number of rolled sheets;

The Best Mode for Carrying the Invention

This invention will be explained hereinafter on the basis of examples shown in drawings.

Fig. 1 illustrates a six high rolling mill according to the present invention.

Referring to Fig. 1, a housing 1 is provided with pairs of upper and lower work rolls 2, intermediate rolls 3 and backup rolls 4, respectively. Both work rolls 2 are capable of being shifted in mutually opposite directions along their axes by means of shifting units 5 for each of them. Both intermediate rolls 3 are also capable of being shifted in mutually opposite directions along their axes by means of other shifting units 6 for each of them.

Each of the backup rolls 4 is constituted by a so-called plain roll having a constant barrel diameter throughout its entire length, and each of the intermediate rolls 3 is constituted by a roll having a barrel length longer than that of the backup roll and a "S" shaped roll crown.

It is preferred that the "S" shaped roll crown of the intermediate rolls has a difference between maximum and minimum roll diameters not larger than 1mm.

The intermediate rolls 3 with such a roll crown are arranged in mutually opposite positions as shown in Fig. 1 and shifted in mutually opposite directions between the maximum and minimum shift positions shown in Fig. 3(a) and (b) by means of shifting units 6.

In the minimum shift position shown in Fig. 3 (a), one barrel end 3a of the intermediate roll 3 is just aligned to one barrel end 4a of its backup roll 4, while in the maximum shift position shown in Fig. 3(b) the other barrel end 3b of the intermediate roll 3 is just aligned to the other barrel end 4b of its backup roll 4. Thus the intermediate rolls contact their respective backup rolls along the full length of the backup rolls at the maximum and minimum shifted positions.

As can be seen from Figs. 1 and 3, the work rolls 2 are plain rolls having a constant diameter and having the same barrel length as that of the backup rolls.

Referring to Fig. 1, in the rolling mill with rolls 2, 3 and 4 arranged as mentioned above, each of the work rolls 2 is connected to a reduction gear 10, attached to a motor 9, successively by means of a spindle 7 and a pinion stand 8. In this case, the shifting position of the work roll 2 (caused by the shifting unit 5, connected to the work roll 2 through the spindle 7 and the pinion stand 8) is detected by a position detecting unit 11 which can be, for example, a magnet scale, and the shifting position of the intermediate roll 3 (caused by the shifting unit 6 connected to the intermediate roll 3) is detected by another position detecting unit 12 which can be also, for example, a magnet scale.

Incidentally, in the figure, 13, 14 and 15 denote a rolled sheet as a product, a work roll bender and an intermediate roll bender, respectively, and 16 indicates a load cell.

Fig. 4 is a diagrammatic view of a control system of the rolling mill as described above.

In the figure, 21 indicates an arithmetic unit, and into this arithmetic unit 21 are inputted beforehand rolling conditions in one cycle such as the shape and size of any tapered portion of the work roll 2, the roll crown and size of the intermediate roll 3, the sheet width, the draft at each roll stand, the sheet finish thickness, the target sheet crown, the target sheet shape and the like, and the arithmetic unit 21 calculates the setting values for the shifting amount of the intermediate roll 3 and the bending force of each of the roll benders 14 and 15 on the basis of such information and the cyclic shifting amount of the work roll 2 in order to provide a sheet crown and a sheet shape as required by the target.

On the basis of the calculation result, each of a shifting control unit 22 and a bender control unit 23 controls the operations of the shifting unit 6 and the roll benders 14 and 15 so that the shifting amount of the intermediate roll 3 and the roll bending force are used as setting values to wait for the start of rolling in such a state.

On the other hand, during the rolling, on the basis of feedback signals from a sheet shape detecting unit 24 and a sheet crown detecting unit 25 to the arithmetic unit 21, in order to realize the target sheet shape and the target sheet crown with high accuracy, the arithmetic unit 21 calculates corrected values of the intermediate roll shifting amount and the roll bending force, and the shifting control unit 22 and bender control unit 23 adjust the shift amount of the intermediate roll 3 and the bending force of the roll benders 14, 15 in accordance with the correction values.

When rolling is carried out by the aforementioned rolling mill, especially under the function of the roll crown acting on the intermediate roll 3, the rolling load exerted on the side edge portions of a sheet bar by the work roll can be very effectively lowered. Therefore, in addition to the actions of the roll benders 14, 15, not only the sheet crown can be controlled with high accuracy but by shifting the intermediate roll 3, its control range can be sufficiently extended.

Next, a method to give a roll crown to the intermediate roll 3 will be explained, by way of an example in which a roll crown is given in accordance with an equation of the third order as shown in Fig. 2.

That is, the lower roll profile of the intermediate roll 3 shown in Fig. 2(a) is the same as the curve shown in Fig. 2(b), and this curve can be expressed by the following equation (1). y1(x) = -a[{x - (δ + OF)}/L]3 + b(x/L) where

y₁:: generating line of the roll crown,
a :: coefficient of the third order,
b :: coefficient of the first order,
x :: coordinate of the barrel center,
L :: 1/2 of the barrel length of the intermediate roll,
δ :: shift amount of the intermediate roll (The start point is x = LB.), and
OF:: offset amount in the axial direction.

On the other hand, the upper roll profile of the intermediate roll 3 being in point symmetry to the lower roll profile with respect to the centre point of the mill can be expressed by the following equation (2) wherein y₂ is the generating line of the roll crown. y2(x) = -a[{x + (δ + OF)}/L]3 +b(x/L)

From the aforementioned equations (1) and (2), the gap Δy between the upper and lower rolls is expressed by the following equation. Δy(x) = y1 - y2 = 2·a· δ+OFL 3 xL 2+ δ+OFL 2

Composite roll crown CR formed by the upper and lower intermediate rolls can be expressed by the following equation (4), wherein the mill center is set to be zero (0). CR = Δy(O) - Δy(x) = -6a{(δ + OF)/L}(x/L)2

The maximum shift amount δ_max to give the maximum composite roll crown can be expressed as follows. δmax = L - LB where L_B: 1/2 of the barrel length of the backup roll. In order to make the composite crown of the upper and lower intermediate rolls to be zero when the shift amount is the minimum value of δmin {=-(L - LB)}, the offset amount OF must be as follows. OF = L - LB

In a normal hot rolling process, the minimum crown amount may be when the composite crown of the upper and lower rolls is zero. However, when it is necessary to make the minimum composite crown larger or smaller than zero, offset amount OF using the position where the shift amount of the intermediate roll is zero (x = L) as a starting point, may be determined as follows. OF = C(L = LB) where C is a constant.

In order to reduce the difference between the maximum and minimum diameters of the intermediate roll without changing the composite roll crown, it is effective to use the following equation obtained when equations (5) and (6) are substituted for equation (4). CR = -6a{(1 + C)(L - LB)/L3}·x3 and to make the third order coefficient "a" to be minimum, therefore to make (L - LB)/L³ to be maximum in the aforementioned equation. In order to make (L-L_B)/L³ to be maximum, the following equation is applied. L = 1.5LB

Accordingly, when the barrel length of the intermediate roll is made 1.5 times as long as that of the backup roll, the maximum and minimum diameter differences of the intermediate roll can be made small, that is, when an S-shaped roll crown is formed on the intermediate roll, the grinding amount can be reduced, so that the life of the intermediate roll can be lengthened in the process of roll grinding.

Fig. 5 shows the result of a comparison of the pressure distribution between rolls and the sheet crown when using an intermediate roll of L = 1.1L_B. As shown in Fig. 5, when the barrel length is 1.5L_B (solid line), the work roll is bent along the intermediate roll, so that the sheet crown is reduced as compared with a case in which the barrel crown is 1.1L_B. Also, as shown in Table 1, it is apparent that the maximum pressure is smaller when the barrel length is 1.5L_B, so that it contributes to improve the roll life.

Length of intermediate roll	Line pressure (kgf/mm) between intermediate and backup rolls	Line pressure (kgf/mm) between intermediate and work rolls
1.5L_B	911	986
1.1L_B	1140	1155

[Experimental Example]

Next, the results of an experiment concerning the intermediate roll, especially barrel length, will be explained as follows.

That is the barrel length of the work roll used was 2300 mm, its diameter was 680 mm, the barrel length of the backup roll used was 2300 mm, and its diameter was 1330 mm. The barrel length of the intermediate roll was variously changed in which the third order coefficient "a" of equation (8) was 0.833. Sheet bars, having a width of 1500 mm and a thickness of 5.2 mm, were rolled to a thickness of 4.16 mm, and various investigations were made.

First, Fig. 6 shows the relationship between the ratio (L/L_B) of the intermediate and backup roll barrel lengths, and the maximum pressure between the intermediate and backup rolls. As shown in the drawing, when the ratio (L/L_B) is increased to not less than 1.2 times, the pressure is gently lowered, so that it is apparent that an intermediate roll of long barrel length is favorable.

Fig. 7 shows the contact condition between the intermediate and backup rolls with respect to the ratio of barrel length under the condition that the same sheet crown is obtained. As can be seen from Fig. 7, when the ratio is increased to not less than 1.2 times, the occurrence of a non-contact region can be prevented, and it is effective to improve the sheet thickness accuracy and to inhibit the occurrence of sheet meander and reduction ears.

In general, when a gap is formed between a block installed in a mill housing for shifting an intermediate roll, and a chock of the intermediate roll (this gap is formed due to abrasion caused by the sliding of the intermediate roll, and also due to defective accuracy of the machine), a deflection is generated in the intermediate roll 3 as shown in Fig. 8(a). Fig. 9 shows the relationship between the horizontal deflection amount t and the ratio (L/L_B) of the barrel length of the intermediate and backup rolls under the condition that the aforementioned gap is 3 mm, wherein the maximum displacement amount t between the chocks shown in Fig. 8(b) is defined as the horizontal deflection amount.

As shown in Fig. 9, the more the ratio is increased, the more the horizontal deflection amount is increased. When the horizontal deflection amount is increased, the gap between the upper and lower work rolls is changed and when the horizontal deflection amount of the upper intermediate roll and that of the lower intermediate roll become different, the roll gap between the upper and lower work rolls becomes varied in the axial direction. Therefore the sheet crown and the sheet profile fluctuate during the rolling operation. For that reason, in order to reduce the barrel length ratio, the intermediate roll length is preferred to be short. However, in the case where the horizontal bending amount is to the extent of 0.45 mm, it has little influence on the sheet crown and profile, so that it causes no problem in a normal rolling operation. Further, the aforementioned gap is usually controlled to be not more than 3 mm. Therefore, it is apparent that when the barrel of the intermediate roll is not more than 2.5 times as long as the backup roll, the rolling can be carried out.

[Specific Example]

A comparative example will be explained as follows in which a crown distribution with respect to the number of rolled sheets and others were investigated when using a rolling mill according to the present invention and also when using a conventional rolling mill.

Rolling Mill of the Present Invention

In a rolling mill train in which six high rolling mills structured as shown in Fig. 1 were arranged in three rolling stands in the rear stage, sheet bars of 900 to 1600 mm width and 40 mm thickness, were rolled to produce a low carbon steel thin sheet of 1.6 to 3.2 mm finished thickness, and then the sheet crown was measured every 5 coils at a position spaced from the edge by 25 mm.

In this case, the barrel length of the work rolls was 2300 mm, that of the intermediate roll was 3450 mm, and that of the backup roll was 2300 mm. Also, the difference between the maximum and minimum diameters of the intermediate roll was 0.8 mm, and the intermediate roll was shifted within a range from 0 mm to 700 mm.

Rolling Mill of the Prior Art

In a rolling mill train, six high mills were arranged in three rolling stands in the rear stage including the final rolling stand. Each six high mill had work rolls, intermediate rolls and backup rolls, all of them being plain rolls and all having a barrel length of 2300 mm. The intermediate rolls were shifted, and rolling operations were carried out in the same manner as the rolling mill of the invention, and the sheet crown was measured in the same manner.

Results of Experiments

Results of measurement are shown in the graph of Fig. 10.

According to the results shown in Fig. 10, when the rolling mill of the present invention was used, it is apparent that it was possible to carry out a highly accurate sheet rolling operation to obtain a sheet crown close to the target sheet crown out even when the target crown was changed. In this case, the rolling schedule with respect to the sheet width of the rolling mill of the present invention was set to be the same as that of the rolling mill of the prior art.

The frequency of occurrence of reduction ears, the accuracy of the sheet thickness, and the average value of the sheet crown are shown in Table 2 in the case where 100,000 tons of sheets were rolled in a thin cycle rolling schedule using the aforementioned rolling mills of the invention and the conventional rolling mills. According to this table, both the sheet thickness accuracy and the pass property (decrease in the occurrence of reduction ears) of the rolling mill of the invention are far superior to those of the conventional rolling mill.

	Average crown E₂₅ (µm)	Sheet thickness accuracy lσ (µm)	Frequency of ears (time)
Inventive rolling mill	40	±46	2
Conventional rolling mill	45	±60	11

In the rolling mill of the invention as described above, it is preferable to supply lubricant to gaps between the backup and intermediate rolls and/or the intermediate and work rolls.

Referring to Fig. 11, lubricant supplying nozzles 26 are arranged to direct lubricant from these nozzles to a gap between the backup roll 4 and the intermediate roll 3 and to a gap between the intermediate roll 3 and the work roll 2. The lubricant is supplied to the lubricant supplying nozzles 26 by supply pipes 29 from a lubricant tank 27 by means of a pump 28. Furthermore, coolant is supplied to the intermediate rolls 3 and the work rolls 2 from cooling nozzles 32 by coolant supply pipes 31 and a coolant pump 30. The preferred lubricant is a highly concentrated emulsion of basic oil including a high pressure agent, but when the lubricant is also used for cooling the rolls, a lubricant having a low concentration may be used.

Referring to Fig. 12, the distance between the lubricant supply nozzles 26 for the barrel portion of the intermediate roll 3 having the larger diameter is preferably smaller than that for the barrel portion having the smaller diameter to increase the amount of lubricant supplied. Instead of increasing the amount of lubricant supplied, the concentration of the lubricant may be varied in the axial direction of the intermediate roll to obtain the same effect as mentioned above.

The rolling mill shown in Fig. 1 was used to roll the sheet bars mentioned above using a 10% emulsion as lubricant and, as coolant, industrial water in a manner as shown in Fig. 11 and at least 120 strips were rolled without roll seizure occurring. In a comparison example, the sheet bars were rolled in the same manner as mentioned above but using only industrial water as coolant. In this case roll seizure occurred on the work roll and the intermediate roll when 100 strips had been rolled and the rolling operation was stopped.

In a rolling mill including an intermediate roll provided with roll crown, distribution of the contact pressure between rolls is varied to vary the bending of the work roll, thereby controlling the sheet crown, and therefore the shape of the sheet. Thus, the amount of crown control is not varied by the change of rolling load. Accordingly, when the diameter of the work roll is small, the deflection amount of the center line of the work roll is greatly varied so that the amount of crown control generated by shifting the intermediate roll becomes large. While, when the diameter of the work roll is large, change in the deflection amount of the center line of the work roll is small so that the amount of crown control generated by shifting the intermediate roll becomes small.

Results of test carried on rolled sheets of 1500 mm width with respect to the diameter of the work roll and the amount of crown control are shown in Fig. 13. As can be seen from Fig. 13, when the diameter of the work roll is small, preferably not more than 700 mm, the amount of crown control becomes large, but when the diameter of the work roll is smaller than 400 mm, the amount of horizontal bending of the work roll becomes large and the roll profile becomes wrong. Thus the work roll is difficult to drive and the affect caused by bending of the work roll is decreased. Accordingly, a work roll of a diameter of at least 400 mm is desirable.

[Example 2]

Fig. 14 shows a rolling mill having improved rigidity by extending the roll barrel of the work roll 2 to make its barrel length longer than that of the intermediate roll 3 in the six high rolling mill shown in Fig. 1. The mill rigidity of the rolling mill is determined by the size of the gap between the work rolls when the rolling load is changed. The gap is influenced by the deflection of the backup rolls, the elastic deformation of the housing etc., and the flat deformation between the rolls. When the barrel length of the work roll is long and then the region contacting the work roll and the intermediate roll is long, the mill rigidity of the rolling mill is great since the contacting pressure between the rolls is smaller than in the case of a shorter contacting region even if the rolling load is changed. Therefore, when the barrel length of the work roll is long, even if the sheet passes in a position deviated from the center of the rolling mill, the variation in the pressure between the rolls is small and the difference between the amounts of deformation at the left and right side with respect to the center line of the sheet is small. Accordingly the use of a work roll having a long barrel length is effective for preventing sheet meander and the occurence of reduction ears.

It should be noted that a preferred range of the barrel length of the intermediate roll is 1.5∼2.5 times as long as that of the backup roll as described above, and this is for substantially the same reason.

A comparative test will be explained in connection with the crown distribution with respect to the number of rolled sheets etc. which were investigated using the rolling mill according to this example and also using a conventional rolling mill.

In a hot finish rolling mill train in which the six high rolling mills structured as shown in Fig. 14 were arranged in three rolling stands in the rear stage, sheet bars were rolled under the same conditions as in the aforementioned Example 1, and then the sheet crown was measured every 5 coils at a position spaced from the edge by 25 mm.

In this case, the barrel length of the work roll was 3400 mm, that of the intermediate roll was 3000 mm, and that of the backup roll was 2300 mm. Also, the difference between the maximum and minimum diameters of the intermediate roll was 0.8 mm, and the intermediate roll was shifted within a range of from 0 mm to 700 mm.

The specification of the conventional rolling mill used in this comparative test was the same as in the case of Example 1.

Results of Experiments

Results of measurement are shown in the graph of Fig. 15. According to the results shown in Fig. 15, when the rolling mill of the present invention was used, it was possible to carry out a highly accurate sheet rolling operation to obtain a sheet crown extremely close to the target sheet crown even when the target crown was changed.

The frequency of occurence of reduction ears, the accuracy of the sheet thickness, and the average value of the sheet crown are shown in Table 3 in the case where 100,000 tones of sheets were rolled in a thin cycle rolling schedule using the aforementioned rolling mill of the invention and the conventional rolling mill. According to this table, both the sheet thickness accuracy and the pass property (decrease in the occurrence of reduction ears) of the rolling mill of the invention are far superior to those of the conventional rolling mill.

	Average Crown E₂₅ (µm)	Sheet thickness accuracy lσ (µm)	Frequency of ears (time)
Inventive rolling mill	45	±38	1
Conventional rolling mill	50	±60	11

[Example 3] Rolling Mill of the present invention

In a cold rolling mill train consisting of four rolling stands in which the six high rolling mills structured as shown in Fig. 1 were arranged in the first rolling stand, sheet bars of 900 to 1600 mm width and 2∼3 mm thickness, were rolled to obtain a low carbon steel thin sheet of 1.6 to 0.5 mm finished thickness, and then the sheet thickness deviation was investigated at a position spaced from the edge by 100 mm.

In this case, the barrel length of the work roll was 2000 mm, that of the intermediate roll was 2700 mm, and that of the backup roll was 2000 mm. Also, the difference between the maximum and minimum diameters of the intermediate roll was 0.8 mm and the intermediate roll was shifted within a range of from 0 mm to 700 mm.

Rolling Mill of the Prior Art

A six high mill was arranged in the first rolling stand and provided with work rolls, intermediate rolls and backup rolls, all of them being plain rolls and having a barrel length of 2000 mm. While the intermediate rolls were shifted, rolling operations were carried out in the same manner as the rolling mill of the invention, and the sheet thickness deviation was measured in the same manner.

Results of Experiments

Results of measurement are shown in the graph of Fig. 16. According to the results shown in Fig. 16, when the rolling mill of the present invention was used, it is apparent that the occurence of edge drop is reduced.

The frequency of occurence of reduction ears and the amount of edge drop are shown in Table 4 in the case where 100,000 tons of sheets were rolled using the aforementioned rolling mill of the invention and the conventional rolling mill. According to this table, both the sheet thickness accuracy and the pass property (decrease in the occurrence of reduction ears) of the rolling mill of the invention are far superior to those of the conventional rolling mill. The amount of edge drop is defined by thickness deviations at positions spaced from the edge by 100 mm and 7.5 mm.

	Amount of edge drop (µm)	Frequency of ears (time)
Inventive rolling mill	12	0
Conventional rolling mill	15	3

When applying the six high rolling mill according to the present invention for cold rolling sheet, in particular for controlling the edge drop in the sheet, since deformation of the sheet in the direction of the sheet width decreases as the sheet passes through the rear stands in the cold rolling mill train, the six high rolling mill should be arranged in the first stand, and preferably six high rolling mills are applied for the rear stands in order from the first stand. The strip sheet is subjected to a tension between the stands of the cold rolling mill train so that meander of the sheet is restrained, but if the hot rolled sheet has a large camber and wedge, reduction ear sometimes occurs owing to the camber and wedge. In the rolling mill of the present invention, however the intermediate roll has a long roll barrel to ensure mill rigidity so that it is possible to prevent the occurence of reduction ear in the sheet.

Next, a six high rolling mill including intermediate rolls having a roll crown which is tapered toward one end or both ends will be described.

[Example 4]

Fig. 17 illustrates a six high rolling mill in which the intermediate rolls 3 are provided with the "S" shape roll crowns, respectively, and the work rolls 2 are provided with one side taper shape roll crowns.

In this rolling mill, when the work rolls 2 are shifted from the position shown in Fig. 18(a) to the position shown in Fig. 18(b), respectively, roll gaps between the tapered portions 2a of the upper and lower work rolls 2 are directly increased at both edge portions of the sheet 13 to be rolled so that edge drop can be reduced. As can be seen from Fig. 19, the edge drop can be modified by regulating the distance EL from the starting point of the tapered portion 2a to the edge of the sheet (referring to Fig. 18) so that the edge drop can be controlled in accordance with a predetermined target amount of edge drop.

A comparative test was carried out in which the crown distribution with respect to the number of rolled sheets etc. were using a rolling mill according to the present invention and also using a conventional rolling mill.

Rolling Mill of the Present Invention

In a rolling mill train in which the six high rolling mills structured as shown in Fig. 17 were arranged in three rolling stands in the rear stage, sheet bars of 900 to 1600 mm width and 40 mm thickness, were rolled to obtain a low carbon steel thin sheet of 1.6 to 3.2 mm finished thickness, and then the sheet crown was measured every 5 coils at a position spaced from the edge by 25 mm.

In this case, the barrel lengths of the work roll and backup roll were 2300 mm respectively, and that of the intermediate roll was 3000 mm. Also, the difference between the maximum and minimum diameters of the "S" shape roll crown formed on the intermediate roll was 0.8 mm, the tapered portion 2a of the work roll was tapered by a 8×10^-3 (0.16 mm/200 mm per diameter) and the intermediate roll was shifted within a range of from 0 mm to 700 mm.

Rolling Mill of the Prior Art

In a rolling mill train six high mills were arranged in three rolling stands in the rear stage including the final rolling stand. Each six high mill was provided with work rolls, intermediate roll and backup rolls, all of them being plain rolls and all having a barrel length of 2300 mm. While the intermediate rolls were being shifted, rolling operations were carried out in the same manner as the rolling mill of the invention, and the sheet crown was measured in the same manner.

Results of Experiments

Results of measurement are shown in the graph of Fig. 20. According to the results shown in Fig. 20, when the rolling mill of the present invention was used, it is apparent that it was possible to carry out a highly accurate sheet rolling operation to obtain a sheet crown extremely close to the target sheet crown even when the target sheet crown was changed. In this case, the rolling schedule with respect to the sheet width of the rolling mill of the present invention was set to be the same as that of the rolling mill of the prior art.

The frequency of occurence of reduction ears, the amount of edge drop, the accuracy of the sheet thickness, and the average value of the sheet crown are shown in Table 5 in the case where 100,000 tons of sheets were rolled. According to this table, both the sheet thickness accuracy and the pass property (decrease in the occurrence of reduction ears) of the rolling mill of the invention are far superior to those of the conventional rolling mill. The amount of edge drop is measured by the difference in sheet thickness at positions spaced from one sheet edge by 100 mm and by 25 mm.

	Average Crown E₂₅ (µm)	Sheet thickness accuracy lσ (µm)	Amount of edge drop (µm)	Frequency of ears (time)
Inventive rolling mill	38	±43	26	6
Conventional rolling mill	50	±60	39	12

[Example 5]

In a cold rolling mill train consisting of four rolling stands in which the six high rolling mills structured as shown in Fig. 17 were arranged in the first rolling stand, sheet bars of 900 to 1600 mm width and 2∼3 mm thickness, were rolled to obtain a low carbon steel thin sheet of 0.5 mm finished thickness, and then the sheet thickness deviation was investigated at a position spaced from the edge by 100 mm.

In this case, the barrel length of the work roll was 2000 mm, that of the intermediate roll was 2700 mm, and that of the backup roll was 2000 mm. Also, the difference between the maximum and minimum diameters of the intermediate roll was 0.8 mm, and the intermediate roll was shifted within a range of from 0 mm to 700 mm.

Rolling Mill of the Prior Art

A six high mill was arranged in the first rolling stand and provided with work rolls, intermediate rolls and backup rolls, all of them being plain rolls and all having a barrel length of 2000 mm. While the intermediate rolls were being shifted, rolling operations were carried out in the same manner as the rolling mill of the invention, and the sheet thickness deviation was measured in the same manner.

Results of Experiments

Results of measurement are shown in the graph of Fig. 21. According to the results shown in Fig. 21, when the rolling mill of the present invention was used, it is apparent that occurring of edge drop was greatly reduced.

The frequency of occurence of reduction ears and the amount of edge drop are shown in Table 6 in the case where 100,000 tons of sheets were rolled using the aforementioned rolling mills of the invention and conventional rolling mills. According to this table, both the sheet thickness accuracy and the pass property (decrease in the occurrence of reduction ears) of the rolling mill of the invention are far superior to those of the conventional rolling mill.

	Amount of edge drop (µm)	Frequency of ears (time)
Inventive rolling mill	3	0
Conventional rolling mill	15	3

[Example 6]

Fig. 22 illustrates a rolling mill similar to the six high rolling mill shown in Fig. 17, except that each of the work rolls 2 is provided with a roll crown tapered towards opposite ends.

A comparative test was carried out in which the crown distribution with respect to the number of rolled sheets etc. were investigated using a rolling mill according to the present invention and also using a conventional rolling mill.

Rolling Mill of the Present Invention

In a hot finish rolling mill train in which six high rolling mills structured as shown in Fig. 22 were arranged in three rolling stands in the rear stage, sheet bars were rolled under the same conditions as in the aforementioned Example 4, and then the sheet crown was measured every 5 coils at a position spaced from the edge by 25 mm.

In this case, the opposite tapered barrel portions 2a and 2b of the work roll were tapered by 0.4×10^-3 (0.08 mm/200 mm per diameter). Also, the difference between the maximum and minimum diameters of the intermediate roll was 0.8 mm, and the intermediate roll was shifted within a range of from 0 mm to 700 mm. The specification of the conventional rolling mill used in this comparative test was the same as in the case of Example 4.

Results of Experiments

Results of measurement are shown in the graph of Fig. 23. According to the results shown in Fig. 23, when the rolling mill of the present invention was used, it is apparent that it was possible to carry out a highly accurate sheet rolling operation to obtain a sheet crown extremely close to the target sheet crown even when the target crown was changed.

The frequency of occurence of reduction ears, the accuracy of the sheet thickness, and the average value of the sheet crown are shown in Table 7 in the case where 100,000 tons of sheets were rolled in a thin cycle rolling schedule using the aforementioned rolling mill of the invention and the conventional rolling mill. According to this table, both the sheet thickness accuracy and the pass property (decrease in the occurrence of reduction ears) of the rolling mill of the invention are far superior to those of the conventional rolling mill.

	Average Crown E₂₅ (µm)	Sheet thickness accuracy lσ (µm)	Amount of edge drop (µm)	Frequency of ears (time)
Inventive rolling mill	40	±40	28	7
Conventional rolling mill	50	±60	39	12

[Example 7]

Fig. 24 illustrates a rolling mill similar to the six high rolling mill shown in Fig. 22, except that the barrel length of the work roll 2 is longer than that of the intermediate roll 3.

A comparative test was carried out in which the crown distribution with respect to the number of rolled sheets etc were investigated using a rolling mill according to the present invention and also using a conventional rolling mill.

Rolling Mill of the Present Invention

In a hot finish rolling mill train in which six high rolling mills structured as shown in Fig. 24 were arranged in three rolling stands in the rear stage, sheet bars were rolled under the same condition as in the aforementioned Example 6, and then the sheet crown was measured every 5 coils at a position spaced from the edge by 25 mm.

In this case, the barrel length of the work roll was 3400 mm, that of the intermediate roll was 3000 mm, and that of the backup roll was 2300 mm. Also, the intermediate roll was shifted within a range of from 0 mm to 700 mm. The conventional rolling mill train used in this comparative test had six high mills arranged in three rolling stands in the rear stage including the final rolling stand. Each six high mill had work rolls, intermediate rolls and backup rolls, all of them being plain rolls and all having a barrel length of 2300 mm.

Results of Experiments

Results of measurement for the sheet crown are shown in the graph of Fig. 25. According to the results shown in Fig. 25, when the rolling mill of the present invention was used, it is apparent that it was possible to carry out a highly accurate sheet rolling operation to obtain a sheet crown extremely close to the target sheet crown even when the target crown was changed.

The frequency of occurence of reduction ears, the accuracy of the sheet thickness, and the average value of the sheet crown are shown in Table 8 in the case where 100,000 tons of sheets were rolled using the aforementioned rolling mill of the invention and the conventional rolling mill. According to this table, both the sheet thickness accuracy and the pass property (decrease in the occurrence of reduction ears) of the rolling mill of the invention are far superior to those of the conventional rolling mill.

	Average Crown E₂₅ (µm)	Sheet thickness accuracy lσ (µm)	Amount of edge drop (µm)	Frequency of ears (time)
Inventive rolling mill	41	±42	25	5
Conventional rolling mill	50	±60	39	12

[Example 8]

Fig. 26 illustrates a rolling mill similar to the six high rolling mill shown in Fig. 24, except that each of the work rolls 2 is provided with a roll crown tapered toward opposite ends.

Rolling Mill of the Present Invention

In a hot finish rolling mill train in which six high rolling mills structured as shown in Fig. 26 were arranged in three rolling stands in the rear stage, sheet bars were rolled under the same conditions as in the aforementioned Example 7, and then the sheet crown was measured every 5 coils at a position spaced from the edge by 25 mm.

In this case, the opposite tapered barrel portions 2a and 2b of the work roll were tapered by 0.8×10^-3 (0.16 mm/200 mm per diameter) and 0.01×10^-3 (0.02 mm/200 mm per diameter), respectively, and the intermediate roll was shifted within a range of from 0 mm to 700 mm. The specification of the conventional rolling mill used in this comparative test was the same as in the case of Example 7.

Results of Experiments

Results of measurement are shown in the graph of Fig. 27. According to the results shown in Fig. 27 when the rolling mill of the present invention was used, it is apparent that it was possible to carry out a highly accurate sheet rolling operation to obtain a sheet crown extremely close to the target sheet crown even when the target crown was changed.

The frequency of occurence of reduction ears, the accuracy of the sheet thickness, and the average value of the sheet crown are shown in Table 9 in the case where 100,000 tons of sheets were rolled in a thin cycle rolling schedule using the aforementioned rolling mill of the invention and the conventional rolling mill. According to this table, both the sheet thickness accuracy and the pass property (decrease in the occurrence of reduction ears) of the rolling mill of the invention are far superior to those of the conventional rolling mill.

	Average Crown E₂₅ (µm)	Sheet thickness accuracy lσ (µm)	Amount of edge drop (µm)	Frequency of ears (time)
Inventive rolling mill	40	±46	24	2
Conventional rolling mill	45	±60	39	11

Claims

A six high rolling mill comprising upper and lower work rolls (2), a pair of intermediate rolls (3) and a pair of backup rolls (4) wherein at least the intermediate rolls and the work rolls are adapted to be shifted in their axial directions, said intermediate rolls are provided with roll crowns which are in a point symmetrical relationship with reference to the centre point of the mill, and said work rolls have roll profiles which are in a point symmetrical relationship with reference to the centre point of the mill characterised in that

the roll profile of one of the intermediate rolls (3) is expressed by the following ternary equation (1): y1(x)= -a[{x-(δ+OF)}/L]3+b(x/L)

where y₁ is the generating line of the crown of the roll,

a is a coefficient of the third order,

b is a coefficient of the first order,

x is the coordinate of the barrel center,

L is 1/2 of the barrel length of the intermediate roll,

δ is the shift amount of the intermediate roll relative to a start point where x=LB, and

OF is the offset amount in the axial direction; in that the roll profile of the other of the intermediate rolls is expressed by the following ternary equation (2): y2(x)= -a[{x+(δ+OF)}/L]3+b(x/L)

where y₂ is the generating line of the crown of the roll; and in that each of the intermediate rolls has a barrel length 1.2-2.5 times longer than that of its backup roll such that the intermediate rolls always contact the backup rolls over the full length thereof at the maximum and minimum shifted positions of the intermediate rolls.
The six high rolling mill according to claim 1, wherein each work roll (2) is a plain roll having a constant diameter.
The six high rolling mill according to claim 1 or 2, wherein each of the upper and lower work rolls (2) has a one side taper-shape roll crown which is tapered from one of the barrel ends towards the other.
The six high rolling mill according to claim 1 or 2, wherein each of the upper and lower work rolls (2) has a two side taper-shape roll crown which extends from an intermediate region of the roll barrel towards the barrel ends.
The six high rolling mill according to any one of claims 1 to 4, wherein the barrel length of each work roll (2) is longer than that of the intermediate rolls (3).
The six high rolling mill according to claim 5, wherein the barrel length of each work roll (2) is 1.4 - 2.5 times longer than that of the intermediate rolls (3).